The present invention relates to the magnetic iron oxide particles containing ferrous, which the particles are fine in size, high in coercive force, and magnetically and chemically stable.
With progressing miniaturization and weight reduction of magnetic recording and reproducing devices in recent years, the necessity has arisen keenly for the higher quality and performance of recording media such as magnetic tapes and discs. Especially demands are heard for higher recording density, higher sensitivity, higher output capacity, and higher magnetic and chemical stability of the magnetic media.
For satisfying such requirements for the magnetic recording media, the magnetic iron oxide particles used for making such recording media are essentially demanded to be fine in size, high in coercive force, and also magnetically and chemically stable.
This fact is referred to in many literatures. For instance, "Development of Magnetic Materials and High Dispersion Techniques for Magnetic Powders" (1982) published by Sogo Gijutsu Center Co., Ltd. states on page 310: "Since the main themes of improvement on magnetic tape performance were the realization of higher sensitivity, higher output capacity and lower noise level, the studies have been directed to the attainment of higher coercive force and smaller size of acicular .gamma.-Fe.sub.2 O.sub.3 particles". Also, Japanese Patent Publication No. 55-6580 states: "There is seen a tendency of recording signals to transfer to the shorter wavelength region in recent years, such a tendency being particularly conspicuous in video cassette recorders. In other words, demands is rising for higher-density recording, higher-output capacity and, in particular, improvement of frequency characteristics and magnetic stability. The properties of the magnetic material for satisfying these requirements for the magnetic recording media are magnetic stability and a high coercive force (Hc)".
Among the known magnetic iron oxide particles, magnetite particles have the advantages that they are high in coercive force and saturated flux density in comparison with maghemite particles, and that when a magnetic recording medium is made by using magnetite particles, such recording medium is hardly electrically charged because of high electroconductivity.
As a typical example of magnetic iron oxide particles having high coercive force, there are known so-called Co-modified magnetic iron oxide particles which have been produced by coating on or substituting the surfaces of magnetite or maghemite particles as precursor particles with a Co compound. In preparation of such Co-modified magnetic iron oxide particles, it is commonly practiced to incorporate ferrous along with Co in the particles, when coating on or substituting their surfaces with a Co compound, for further enhancing the coercive force.
Thus, the magnetic iron oxide particles which are fine in size, high in coercive force, and also magnetically and chemically stable are the most eagerly required in the art at present. However, known magnetite particles or Co-modified magnetic iron oxide particles, although having high coercive force as mentioned above, have the defect that they are magnetically and chemically unstable due to incorporation of ferrous. That is, when the magnetic iron oxide particles containing ferrous are left in the air, ferrous contained therein is oxidized into ferric, thereby deteriorating magnetic properties, especially reduction of coercive force, with the passage of time. This phenomenon tends to become more marked as the particle size becomes smaller.
Recently, the problem is also pointed out that when magnetic iron oxide particles containing ferrous are used as magnetic tape coating material, the color tone of the coat is changed from normal black into dark brown due to oxidation of ferrous in the said particles into ferric to cause an increase of light transmittance, resulting in inducing improper run of magnetic tape in the recording devices such as video deck.
As a result of the present inventors ' extensive studies to provide magnetic iron oxide particles containing ferrous which are free of the said defects and can well satisfy the said requirements, it has been found that magnetic iron oxide particles containing ferrous and having their surfaces coated with double layers of a lower layer of a Zn compound and an upper layer of a Si compound, which have been obtained by mixing and dispersing magnetic iron oxide particles containing ferrous in a solution of a Zn compound to adsorb the Zn compound on the particle surfaces, and then mixing and dispersing the obtained Zn compound-adsorbed particles in a solution of a Si compound to adsorb the Si compound on the Zn compound-adsorbed particle surfaces, are satisfactorily fine in size, high in coercive force, and also magnetically and chemically stable. The present invention was achieved on the basis of this finding.